The invention relates to a vehicle component with at least one sandwich part. It furthermore relates to a method for the production of such a vehicle component. Thus, the invention also addresses the use of a sandwich part as a crash element absorbing kinetic energy in a vehicle.
Traditional crash elements in vehicles, especially in motor vehicles, are formed from plastically deformable materials, usually metals, and transform the kinetic energy of the collision partners into deformation energy. Especially during side-impact collisions, this transformation occurs through bending sills provided in the vehicle floor and in the vehicle side walls and vehicle doors.
The constant striving to reduce the energy consumption of a vehicle has meant that metal parts of vehicles are being replaced increasingly by fiber composites. Fiber composites, such as carbon fiber composites (CFC), may have a high bending stiffness, especially when designed as a sandwich part, but they have an extremely low ductility as compared to metallic materials. Energy-absorbing crash elements consisting of plastics instead of metal therefore do not transform the kinetic energy of the collision partners into ductile deformation energy, but instead into a free surface due to material fragmentation. This fragmentation process, in order to transform as much energy as possible, must be highly efficient; that is, the components directly involved in the collision must undergo fragmentation into the smallest possible pieces. This holds especially for sandwich parts, which are used especially advantageously for load-bearing vehicle parts, but also for large-area parts such as vehicle floors, on account of their good bending stiffness.
The inventor has found that sandwich parts with a layered structure of fiber-reinforced cover layer elements and a core layer element arranged in between, when subjected to large impulse forces parallel to the connection surfaces between the cover layer elements and the core layer element, have a tendency to bulge at first before fragmenting. A core layer element consisting of hard foam in particular has a tendency, when the individual layers are separated from each other in the region of their contact surfaces, to form folds between the detaching cover layer elements before fragmenting. Such a folding process consumes only a very small portion of the kinetic energy applied, and results in a delaying of the fragmentation process which breaks down the major portion of the kinetic energy. Therefore, the effective breakdown of the kinetic energy is delayed and occurs too slowly.
DE 101 28 054 A1 shows and describes a layered composite sheet with openings running in its thickness direction. This known layered composite sheet comprises two carbon fiber-reinforced plastic cover layers, between which is provided a light support layer made of a foam plastic. The openings are fashioned as cylindrical channels which penetrate the layered composite sheet at right angles to its surface. The area of application for such layered composite sheets is, for example, textile machine building or airplane building. A layered composite sheet is to be created in the described manner which for the most part avoids a weakening of the strength or rigidity in the region of the openings. This is accomplished in that an interruption of the reinforcement fibers in the plastic cover layers is avoided by a special manufacturing process in which the reinforcement fibers are led around the openings. This results in a compression of the fiber layer in the regions of the plastic cover layers between the individual openings. Thus, the goal here is to increase the strength of such layered composite sheets.
CH 681 971 A5 concerns a composite with embedded reinforcement. This composite comprises two cover sheets of thermoplastic material, between which is provided a perforated metallic reinforcement layer, i.e., a perforated metal sheet. A composite sheet is to be created with high strength properties, while retaining a deformability, in order to be able to form shaped parts from such a composite or to enable an energy absorption by deformation in the event of a crash, for example. This is accomplished by the perforated metal sheet layer in the plastic composite. The described layered material should be especially suitable for the production of body parts ensuring the highest possible energy absorption under any actions of force. Thus, the purpose of this known layered composite design is to create a light material with high strength properties, which is able under the action of strong force to break down energy by plastic deformation.
WO 2008/049469 A1 shows and describes a sandwich structure made of two textile planar structures for use in resin-bound shell-type components, wherein a filler material is provided between the textile planar structures and wherein the sandwich design made from the two planar structures and the filler material arranged in between is tufted or stitched together. The filler material is formed by an elastic foam plastic layer, so that the sandwich design can be formed three-dimensionally into shell-type parts. A stiffening of such a three-dimensional structure is seen as being undesirable.
The problem which the present invention solves is to design a vehicle component with at least one sandwich part with a layered structure of at least two fiber-reinforced cover layer elements and at least one core layer element provided between two adjacent cover layer elements, so that in the event of impact loads arising, a more effective absorbing of the impact energy occurs by the sandwich part. Furthermore, the present invention provides a method for the production of such a vehicle component.
According to the invention, a vehicle component is provided with at least one sandwich part which forms a crash element absorbing kinetic energy, wherein the at least one sandwich part has a layered structure of at least two fiber-reinforced cover layer elements having a synthetic resin matrix and at least one core layer element provided between two adjacent cover layer elements, and wherein the at least one core layer element comprises channels which pass transversely through the at least one core layer element. The at least one core layer element is formed from a hard foam material or a softwood and the channels provided in the core layer element form predetermined breaking points for the core layer element.
Since the core layer element is formed from a hard foam material or from a softwood, such as balsa wood, or comprises such a hard foam material or softwood, the sandwich part has a high bending stiffness with low weight. The channels which run transversely, preferably at right angles, to the layered structure form predetermined breaking points in the respective layer element, at which the desired fragmentation can occur. The bulging or material folding observed in the experiment with the prior art is prevented in the case of the sandwich part according to the invention in that the predetermined breaking points ensure a fragmentation upon peeling of the layers of the sandwich part already when lesser bending stresses occur in the isolated layer element.
Preferably, at least one of the cover layer elements of the at least one sandwich part also includes channels which pass transversely through the respective cover layer element. In this way, predetermined breaking points are also created in the respective cover layer element. In this variant, the channels provided in the cover layer element also contribute to the improved fragmentation and thus to the energy transformation. In one advantageous embodiment, the channels are formed in both cover layer elements and in the core layer element.
It is also advantageous for the channels to be arranged at least in one region of the sandwich part at uniform spacing from each other. In this way, the failure behavior of the part in this region is designed to be substantially homogeneous. This means that a uniform fragmentation and thus, in the event of a collision, a controlled dissipation of kinetic energy may occur in this region.
The spacing between neighboring channels may in other regions likewise be uniform, but larger or smaller than in neighboring regions of the at least one sandwich part. In this way, regions with fragmentation occurring at different rates can be formed, so that it is possible to steer the fracturing behavior of the sandwich part and thus the intensity of the energy dissipation.
Preferably the cover layer elements comprise carbon fibers which are embedded in the respective synthetic resin matrix in the finished state of the sandwich part. Carbon fibers possess not only a high strength to weight ratio, but are also especially suited to a highly efficient energy-consuming fragmentation.
In one advantageous embodiment, the channels are formed by through holes, preferably through boreholes, which pass through the respective cover layer element or the core layer element. The forming of the channels as through holes entirely passing through the respective layer element in the transverse direction has the benefit that the fragmentation occurs without delay, regardless of the direction in which the respective layer element bulges.
In certain applications it is advantageous for the channels to be formed only in the core layer element and for the openings of the channels to be covered by the respective cover layer element. This variant is preferred when the sandwich part is exposed to external dust or moisture influences, since in this embodiment the cover layer elements close the channels and prevent the penetration of foreign bodies or moisture. Of course, the openings of the channels may be provided in the core layer element and in at least one of the cover layer elements. For example, in addition, only one of the cover layer elements may be closed and have no channel openings if the side of the sandwich part with this cover layer element is exposed to external dust or moisture influences.
In other applications, the channels are formed in the core layer element and in both cover layer elements. In this variant, the channels provided in the cover layer elements contribute to the improved fragmentation.
Of particular advantage is an embodiment of the invention in which the at least two cover layer elements and the at least one core layer element in the sandwich part are mechanically joined to each other by tensile force transmitting elements passing transversely through them.
The tensile force transmitting elements between the cover layer elements hold the layered structure together and brace transverse forces between the cover layer elements which occur under the action of an impact force. Thus, they prevent the cover layer elements from detaching from the core layer element. This slows down the penetration of a collision partner, such as a post, into the sandwich part, because the collision energy has already been dissipated immediately at the start of the collision by fragmentation of the sandwich part, especially by fragmentation of the respective cover layers and the core layer. Thus, the fragmentation occurs successively from the penetration side of the collision partner and continues steadily with increasing depth of penetration, the rate of penetration of the collision partner being effectively slowed down by the energy dissipation. Thus, thanks to the cohesion of the layered structure by way of the tensile force transmitting elements, it is ensured already at the start of the penetration of the collision partner that the highly effective energy-consuming fragmentation of all elements of the layered structure involved, for example both the cover layer elements and the core layer element, commences at once. The depth of penetration of the collision partner is therefore significantly less in the sandwich part of the vehicle component according to the invention for a given kinetic energy than in a traditional sandwich without the tensile force transmitting elements according to the invention, in which significantly less energy per unit of penetration is dissipated on account of a separating of the individual layers.
It is advantageous for the tensile force transmitting elements to be led through the channels. In this way, the failure behavior can be steered by way of channels attached in a defined way, wherein the corresponding design is not affected by additionally provided stitching holes.
Preferably, the tensile force transmitting elements are fixed by the synthetic resin matrix in or on the respective cover layer element. This accomplishes an especially effective binding of the tensile force transmitting elements to the respective cover layer, so that large tensile forces between the cover layers can be braced by use of the tensile force transmitting elements.
It is also of advantage for the tensile force transmitting elements to be formed by wires or threads which are introduced into the layered structure by tufting or stitching and which are fixed in or on the cover layer elements. This embodiment has the advantage of being produced in a simple and economical way.
This method according to the invention for the production of a vehicle component according to the invention with at least one sandwich part forming a crash element absorbing kinetic energy is characterized by the steps of:                providing a layered structure of at least one core layer element formed from a hard foam material or a softwood and at least two cover layer elements, which have fibers, especially carbon fibers, and a synthetic resin matrix and which are arranged on two sides of the core layer element facing away from each other;        introducing channels as predetermined breaking points in at least one of the cover layer elements and/or in the at least one core layer element, which pass transversely through the respective cover layer element or the core layer element, before or after the forming of the layered structure and before or after the crosslinking and curing of the respective synthetic resin matrix.        
The introducing of the channels may occur prior to the juxtaposing of the layer elements of the layered structure or after the juxtaposing, depending on whether all layer elements are to be penetrated by the channels or only individual layer elements. If the introducing of the channels is done after the juxtaposing of the layer elements, i.e., the channels penetrate all layer elements of the layered structure, the channels may be introduced before or after the crosslinking and curing of the respective synthetic resin matrix.
Preferably the introducing of the channels is done by needling of the respective layer element. In this way, a fast processing by machine is possible.
An especially preferred application of the method according to the invention occurs in the production of vehicle components in which a sandwich part produced by the method according to the invention is used as a crash element absorbing kinetic energy of the vehicle. This application of the method according to the invention and the associated use of such a sandwich part in the vehicle component according to the invention combine the benefit of light vehicle construction with a highly effective crash safety.
Preferably, the introducing of the channels is done by needling of the respective layer element.
It is also of particular benefit for the layers of the layered structure to be mechanically joined to each other by introducing and fixing tensile force transmitting elements transversely to the individual layers. This accomplishes the already described strengthened mechanical cohesion of the individual layers.
The introducing and fixing of the tensile force transmitting elements in the layered structure transversely to the individual layers is done immediately after the superposing of the individual layer elements, i.e. before the layered structure is processed further into a hard and firm sandwich material. Semifinished fibers used for the cover layers may be fabrics, scrims, or fiber mats, but the spraying on of fibers is also not ruled out. Preferably, semifinished fibers such as prepregs or rovings are used to form the cover layers.
Preferable is a variant of the method according to the invention in which the step of introducing tensile force transmitting elements is done by stitching of the layers of the layered structure or by tufting of the layers of the layered structure, wherein in the case of tufting the resulting loops are glued in a following step to the synthetic resin matrix of the associated cover layer element or embedded in the latter. A stitching can be done with traditional industrial machinery or stitching robots. The tufting can also be done by means of traditional machines from textile processing. The loops created during the tufting are glued in a following step to the synthetic resin matrix of the associated cover layer element or embedded in the latter. In this way, the loop side of the tufted material is also firmly joined to the cover layer there in that the loops are embedded in the synthetic resin matrix or glued to it.
It is especially advantageous to perform the step of crosslinking and curing of the respective synthetic resin matrix of the cover layers only after the step of introducing the tensile force transmitting elements into the layered structure. This has the advantage that the tensile force transmitting elements can still be incorporated in the soft and uncured cover layer elements, so that only slight forces must be expended for this. During stitching or tufting, this brings the advantage that traditional industrial stitching machines from textile processing may be used for these steps of the method. Moreover, this configuration of the method according to the invention ensures that the tensile force transmitting elements, such as wires or threads, are firmly connected to the synthetic resin matrix of the cover layers.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.